Various types of voltage regulator circuits using transistors as a variable resistance element have been proposed. One such voltage regulator circuit is described in the publication "Electronics", vol. 35, Nr. 44, 1962, pages 48-50 which disclose a voltage regulator with a NPN control transistor; and "Electronics", vol. 38, Nr. 23, 1965, pages 109-110, illustrating a voltage regulator with a PNP transistor.
NPN transistors have a saturation voltage of about 1V. In order to be able to obtain regulated output voltages even though the input voltage is only slightly above the desired, regulated output voltage, for example to obtain a regulated output voltage of 5 V even if the input has dropped to 5.5 V, it is necessary to use PNP transistors which have saturation voltages which can be as low as 200 mV, depending on the type of transistor being used. PNP transistors connected in voltage regulator circuits have the tendency to oscillate, which makes the circuit unstable and thus inhibits use of PNP transistors in voltage regulated circuits in many applications where stability is of primary importance. One such application, for example, is to provide a regulated output voltage for calculating or computer-type circuits, for example, micro-processor circuits in automotive vehicles, in which the supply voltage can vary widely. A typical application is, for example, the type of computer described in U.S. application Ser. No. 660,858, filed Feb. 24, 1976, Mayer, now U.S. Pat. No. 4,082,069 assigned to the assignee of the present application.